How about a hint?
Alder (Alnus)
Apple and Pear (Pyrus)
Ash (Fraxinus)
Aspen and Cottonwood and Poplar (Populus)
Basswood (Tilia)
Birch (Betula)
Buckeye (Aesculus)
Buckthorn (Rhamnus)
California-laurel (Umbellularia)
Catalpa (Catalpa)
Cedar (see "True Cedar")
Cherry and Plum (Prunus)
Chestnut (Castanea)
Chinkapin (Castanopsis)
Cottonwood and Poplar and Aspen (Populus)
Cypress (Cupressus)
Dogwood (Cornus)
Douglas-fir (Pseudotsuga)
Elm (Ulmus)
Fir (see "True Fir")
Filbert and Hazel (Corylus)
Giant Sequoia (Sequoiadendron)
Hawthorn (Crataegus)
Hazel and Filbert (Corylus)
Hemlock (Tsuga)
Honeylocust (Gleditsia)
Holly (Ilex)
Horsechestnut (see "Buckeye")
Incense-cedar (Calocedrus)
Juniper (Juniperus)
Larch (Larix)
Locust (Robinia)
Madrone (Arbutus)
Maple (Acer)
Mountain-ash (Sorbus)
Mountain-mahogany (Cercocarpus)
Oak (Quercus)
Oregon-myrtle (see "California-laurel")
Pear and Apple (Pyrus)
Pine (Pinus)
Plum and Cherry (Prunus)
Poplar and Aspen and Cottonwood (Populus)
Redcedar/Arborvitae (Thuja)
Redwood (Sequoia)
Russian-olive (Elaeagnus)
Spruce (Picea)
Sweetgum (Liquidambar)
Sycamore (Platanus)
Tanoak (Lithocarpus)
True Cedar (Cedrus)
True Fir (Abies)
Walnut (Juglans)
White-cedar (Chamaecyparis)
Willow (Salix)
Yellow-poplar (Liriodendron)
Yew (Taxus)